The present invention refers to a mechanism for transforming a rotational movement into a linear movement, and hereinafter called linear transformation mechanism. It is especially applicable within the aerospace industry.
Aerospace exploitation has required the advance of a new technology of high precision and reliability. Engineering has had to face problems such as the deployment of solar panels, craft coupling, etc., in which high mass restrictions and very strict specifications prevail.
Currently, one of the most demanding fields is the deployment of antennas after launch of a vehicle and subsequent positioning of the antenna in a continuous manner, for which high precision and highly efficient linear movements, originating from rotational movements, are necessary.
Systems existing on the market for transforming a rotational movement into a linear movement offer excellent load and efficiency ranges, but not in combination with feed rates per revolution in the order of tens of microns and not in the combination of various mechanisms of interest to comply with these requirements, due to the consequent weight gain, volume increase, and loss of efficiency and reliability.
In applications of linear transformation mechanisms, two clear tendencies can be distinguished, namely those giving precedence to a very small feed per revolution for operations of adjustment or trimming, orientation, guidance, . . . etc. with high efficiency, and those giving priority to the ability to move large loads and their irreversibility for the deployment of appendages (booms, antennas . . . ) and their maintenance in the final position.
Mechanisms with the purpose described not only have an application within the aerospace field, but may also provide a good choice for use in a machine tool, which requires great precision in linear positioning, or even for use in lifting mechanisms, given their load capacity, due to the great reduction they provide.
Basically, the systems developed until now for the orientation of antennas start from small revolutions caused by “stepper” and geared motors of a very different nature (Harmonic Drive, Planetary Systems, . . . ). The angle most suitable for emitting and receiving signals in antennas on satellites, aircraft . . . , is directly oriented in this way.
A new line followed by the applicant consists in adjustment or trimming systems starting from the rotation of a plane according to predetermined axis systems. The difference in this case is in the concept used to achieve the precise and irreversible rotational movement.
The concept is to fix a point of the plane by means of a ball joint, allowing rotation thereon. On the other hand, two other ball joints are symmetrically joined to the plane, but they have vertical movement ability. Angles of rotation of a magnitude much lower than previous systems are thus obtained.
To obtain upward or downward movement of the latter two points, linear transformation reducing systems fed by the rotation of a “stepper” motor are used. Linear transformation systems are also applicable in deployment mechanisms, usable in equipment and systems requiring prior firm structural fastening. Typical examples include antennas, deployable appendages such as solar panels and deployable radiators, aeronautics, military equipment, etc.
These components should be strongly anchored to the structure of the satellite to survive launch loads on them, whereas in order to be operative, they must be in a different configuration, decoupled, distanced or disconnected from the structure of the satellite. Appendages such as antennas, radiating solar panels, experiments, protective covers, etc., may have to be deployed once the vehicle is in an operative position.
Basically, three types of mechanisms currently exist which can be compared in some aspects with the proposed transformation mechanism. These are known as roller screws, harmonic screws and harmonic drive.
Roller screws imply a step forward with respect to ball screws for some applications. Ball recirculation screws are characterized by their high efficiency in the transformation of movement. This advantage is due to the use of balls between the nut and the screw for transmitting stresses through rolling and not through sliding, as in the most conventional systems. Nevertheless, since the balls need axial space, they have limited pitch reduction. Replacement with rollers allows maintaining the rolling contact together with the longitudinal decrease between thread faces, making feeds per revolution as small as half a millimeter possible. A very high efficiency and a very low feed per revolution ability are achieved with these systems, but they are not low enough for applications of the type set forth.
“Harmonic screw”, encompasses all mechanisms which use a deformable screw and a nut to achieve a very reduced pitch in the transformation of rotational into translational movement. In all cases, very accurate positioning is sought. But, the efficiency of actuation is not taken into account, nor is the design of the screw and the wave generator for providing the mechanism with a large load capacity provided for. A mechanism of this type is disclosed in U.S. Pat. No. 2,979,964. U.S. Pat. No. 4,557,153 discloses a type of harmonic screw having rollers which each push against one point of the deformable screw so as to achieve a contact zone with the nut. This gives little stability to the assembly and reduces its load capacity. Furthermore, those rollers do not have balls or rollers of smaller diameter facilitating the rolling and favoring high efficiency of the mechanism. Very reduced pitches are obtained with this type of mechanism, but they do not achieve high efficiency nor a large load capacity.
Finally, harmonic drive consists of a simple mechanism with which a great reduction and high efficiency and load capacity are obtained. But it provides a reduction from rotational movement to rotational movement, i.e. it constitutes a reducer, and there is meshing between its parts at two points. The working principle consists of a flexible ring which, by means of a slightly elliptical bearing, is cyclically deformed to achieve meshing with an outer, non-deformable ring, wherein the reduction increasing as the difference in diameter between the flexible ring and the non-deformable ring decreases.
Meshing between the flexible ring and the non-deformable ring is equivalent to pure rolling due to friction between the elements mentioned. As the outer non-deformable ring is fixed, the flexible ring will be the one to roll on its interior perimeter. Conceptually, this system is very suitable due to its high reduction and compact form. Nevertheless, the ellipsoidal bearing, known as Wave Generator, offers a very poor support for the typical axial loads of a linear transformation mechanism.